It is a conventional practice in an engine equipped with an exhaust gas turbine turbocharger to extract part of intake air supplied to the intake air compressor of the exhaust gas turbine turbocharger, or part of intake air compressed by the intake air compressor, and to feed this air to a backside space of the exhaust gas turbine or bearing parts, etc (hereinafter referred to as “seal air”), for the purposes of preventing ingress of exhaust gas from the engine into the intake side of the turbocharger, balancing the thrust from the exhaust gas turbine of the turbocharger, and cooling the backside of the intake air compressor and the exhaust gas turbine. The seal air is released to atmosphere after having served the purposes.
For example, Patent Document 1 (Japanese Patent Application Laid-open No. H6-346749) discloses a configuration shown in FIG. 5, in which low-temperature air on the secondary side of a scavenging chamber 03 for a main engine, after having been compressed by a turbocharger 01 and cooled thereafter by a cooler 02, is supplied to a side face of a turbine disc provided on the turbine side 04 of the turbocharger 01 as seal air, so as to cool the turbine blades and the turbine disc parts in addition to the original functions (of balancing the thrust and providing seals for oil and gas).
Patent Document 2 (Japanese Patent Application Laid-open No. H11-117753) discloses, too, a configuration in which compressed air from the compressor of a turbocharger is directed through a seal air passage to the back of the turbine to provide a balance for the rotor shaft, to prevent ingress of exhaust gas, and to provide a seal for preventing leakage of lubricating oil, and in which a valve is attached midway of the seal air passage to automatically open this passage when the air pressure on the compressor outlet side is higher than the pressure in the backside space of the turbine.
Meanwhile, for a gas engine that uses a low energy gas such as coal mine methane gas as fuel gas, a pre-mixing intake system is known, for example as shown in Patent Document 3 (Japanese Patent Application Laid-open No. 2009-144626), in which part or all of fuel gas is pre-mixed to intake air before the turbocharger, so that this gas mixture is compressed by the turbocharger and fed into the cylinders.
Patent Document 1: Japanese Patent Application Laid-open No. H6-346749
Patent Document 2: Japanese Patent Application Laid-open No. H11-117753
Patent Document 3: Japanese Patent Application Laid-open No. 2009-144626
However, in a gas engine with a pre-mixing intake system in which part or all of fuel gas is pre-mixed to air before the turbocharger as disclosed in Patent Document 3, and in particular, in a gas engine where fuel gas such as methane is pre-mixed to air, if the intake gas charged to the engine is utilized as seal air and partly supplied to the turbocharger as shown in Patent Documents 1 and 2, there is a possibility that methane gas may contact high-temperature parts or portions inside the turbocharger, since the intake gas contains methane.
Moreover, releasing the seal air to atmosphere after it has been supplied to the backside space of the turbine and bearing, etc, means releasing part of the fuel gas to atmosphere, which causes an adverse effect on the air environment, as methane gas has a significant greenhouse effect, and which may also deteriorate the fuel consumption efficiency.